Do shallow injection wells contribute to the triggering of the 2020 M5.0 Mentone earthquake in the Delaware Basin, Texas?

The M5.0 Mentone earthquake that occurred in the Delaware Basin, Texas on March 26, 2020 is one of the largest induced earthquakes recorded in the central US. A previous study shows that changes in Coulomb failure stress (CFS) due to the nearby deep injection wells within the highly permeable Ellenburger group (~80 kPa) may have triggered the M5.0 earthquake in the basement, assuming the presence of hydraulic connection between the two layers. The shallow injection wells in the region, on the other hand, contributed much more significant injection volume in the shallower sandstone layer, about five times larger than that of the deep injection. In this study, we investigate, despite the shallower depth, the potential contribution of these wells to the triggering of the M5.0 event and the possible physical mechanisms. First, we perform source mechanism inversion on local seismicity and grouped events into several clusters considering their time and proximity from the M5.0 event. Based on earthquake relocation results from HypoDD, we fit a south-facing normal fault plane with a strike of 81o and dipping of 52o. A fully coupled poroelastic stress modeling is then employed to calculate CFS at the hypocenter location within the basement rocks due to shallow injection wells, assuming no hydraulic connection. We consider a subset of shallow wells within 25 km to the northeast of the mainshock that account for ~40% of the total shallow injection volume. The resulted CFS is ~20 kPa and is predominantly coupled poroelastic stress contribution. This value is comparable to estimates made in other studies that have triggered seismicity on faults well aligned with the local stress field in the central US, and since we only consider a subset of shallow wells, the total poroealstic stress contribution from shallow injection wells could be even larger and of similar order to the CFS from the deep wells on a sealed isolated fault in the basement with no hydraulic connections. Our results highlight the importance of understanding the geologic settings near injection operations and considering the effects of wells at all depths, as well as all possible associated triggering mechanisms when assessing seismic hazard posed by industrial operations.